The present invention relates to a new hydraulic binder comprising a hydraulic active amorphous calcium silicate phase that may contain embedded some residual wollastonite. The hydraulic binder is produced by a combination of heating and cooling operations.
Concrete is one of the most worldwide used manufactured materials. Cement, in particular ordinary Portland Cement (OPC) is the component of concrete responsible for its strength when reacting with water. The present world production of OPC is around 4 billion metric tons per year.
Herein, the following abbreviations, which are conventional in the art, are used, unless otherwise stated:                C represents CaO (calcium oxide);        H represents H2O (water);        S represents SiO2 (silica);        A represents Al2O3 (alumina);        F represents Fe2O3 (iron (III) oxide);        CSH represents an amorphous calcium silicate hydrate, which results from the hydraulic reaction;        Amorphous means a non-crystalline solid phase;        C3S represents tricalcium silicate (3CaO.SiO2), known as alite;        C2S represents dicalcium silicate (2CaO.SiO2), wherein belite is any of the allotropic forms of C2S;        C3S2 represents 3CaO.2SiO2, known as rankinite;        CS represents calcium silicate (CaO.SiO2), wherein wollastonite is any of the allotropic forms of CS;        CH represents calcium hydroxide Ca(OH)2, known as portlandite; and        BAT means best available technology.        
Further, herein the expression “hydraulic binder” means a compound or composition which sets and hardens in the presence of water by hydration, resulting in a solid material. The hydraulic binder is in particular cement, and the solid material is in particular concrete. The expression “latent hydraulic” means the property of a compound or composition to become hydraulic active when mixed with a hydraulic active phase. This means that the compound or composition is not hydraulic in itself but would become so when it is present together with a hydraulic active phase and is exposed to calcium-rich water solutions which triggers that property leading to setting and hardening of the material. Latent hydraulic compounds modify the hydration products formed and as a result modify the properties of cement pastes, mortars and concretes. Also the expressions “highly amorphous” or “essentially amorphous” refer to a material mainly composed of an amorphous phase, i.e. having residual or none crystalline fraction in its constitution.
The best available technology (BAT) for the industrial production of cement uses a well-established two-step technology. In modern plants, the first step is carried out continuously in a rotary kiln, fed with limestone, different silica content materials and fuel (usually “pet coke”, coal or natural gas) producing, at around 1450° C., a clinker composed of about 75% of alite (C3S) (this amount could range from 55 to 78% for conventional OPC clinker), which is afterwards cooled down in a grate or satellite cooler before being stored. The alite (C3S) is the silicate most responsible for the good hydraulic behavior of the material. In the second step the clinker is ground, generally to a Blaine specific surface between 3000 and 3500 cm2/g and eventually mixed with other materials aiming at different corrections and other purposes.
The concept of the existing cement BAT is based on the production of an alitic (C3S) clinker which requires around 1250 kg of limestone per ton of clinker and kiln temperatures around 1450° C., in spite of the use of some fluxes. After grinding, the alite crystals react with water, forming a silicate gel (poorly crystalline calcium silicate hydrate)—CSH—generally with a C/S molar ratio between 1.7 and 1.8 and, simultaneously, a significant amount of portlandite CH (Ca(OH)2). The strength of the cement stone is determined by the structure and chemical composition of the CSH gel, which, at 28 days, represents 40 to 50% in weight. Portlandite generally represents 20 to 25% in weight and contributes to the pH value of the material, but with regard to strength it is an undesired by-product.
Due to the use of limestone as the source of calcium and the high temperature required for the clinkerization process to obtain C3S, the ecological footprint of this BAT for the industrial cement production is quite high, namely as regards CO2 emissions (over 800 kg per ton of clinker), derived both from the decarbonation of the limestone (approximately 60% of the emissions) and the burning of the fuel (remaining 40% of the emissions). As a result, the cement industry is today responsible for more than 5% of all worldwide anthropogenic emissions of CO2.
Due to the fact that OPC is a very versatile, easy to use, durable and relatively inexpensive building material its application is an important element for the social and economic development and well-being of today's society.
Designing and developing a hydraulic binder matching the technical, economic and workability qualities of OPC, and allowing a reduction of the ecologic footprint, namely CO2 emissions, represents simultaneously a great challenge both to the technical research and development and to the fulfilment of the social responsibility obligations of the world cement industry.
Over the last decade the cement industry tried to respond to this challenge using alternative raw materials and fuels that could result in decreasing the CO2 emissions. Some approaches target the partial or total substitution of calcium for other elements with impact on the reduction of CO2 content in the raw materials. Others try to reduce the amount of calcium required developing belitic clinkers. Still others try to develop alternative non-clinker technological routes.
Another subject that has also been object of some attention is the utilization of SCMs (supplementary cementitious materials), such as metakaolin, fly-ashes or slag. SCMs by their selves do not have interesting hydraulic properties being classified as latent hydraulic materials, meaning that they have little hydraulic activity when mixed with pure water, which is why these materials are often mixed with Portland cement (that acts as an activator) to obtain a hydraulic binder able to set and harden when mixed with water. Blends of cement with slag have already been found to have excellent durability as well as compressive strengths comparable to or higher than OPC. In fact, the hydration of slag leads to the formation of calcium silicate hydrates (CSH) with a specific pore size distribution that reduces the permeability of the resulting paste, particularly at early ages.
Representative examples of the state of art are three particular products which are described in patent literature as follows: i) a hydraulic binder being a cement based on a Belite-Calcium-Sulfoaluminate-Ferrite (BCSAF) clinker, which is a clinker with low or no content of alite, described in e.g., U.S. Pat. No. 8,177,903 B2, U.S. Pat. No. 8,317,915 B2, or US 2012/0085265 A1; ii) a hydraulic binder being based on the same raw materials as used in “classical” cement production but using a lower molar Ca/Si ratio, described in e.g., DE 10 2007 035 257 B3, DE 10 2007 035 258 B3, DE 10 2007 035 259 B3, DE 10 2005 037 771 B4, or DE 10 2005 018 423 A1; and iii) a hydraulic binder comprising a ground blast furnace slag, described in EP 2507188 A1, CA 2782232 A1, CN 102666426 A, U.S. Pat. No. 8,328,931 B2, US 2012/0234209 or WO 2011/064378 A1.
On a world scale, the investment of the cement industry in the existing BAT for OPC production is considerable, creating an important economic constraint to a drastic alteration of the existing technology.
Thus, the problem underlying the present invention was to provide hydraulic binders and a process for their production using the traditional clinker route and producing less CO2 emissions than in conventional cement production.